>Rich Measures wrote:
>
...snip...
>Don't need to - I'm sure you're correct that those resonances exist, and
>that they need to be suppressed.
The grid and anode VHF resonances can Not be suppressed----slightly
moved, yes.
>What I'm still not convinced about is
>the need to use nichrome suppressors.
According to Wes' (N7WS) tests on otherwise-similar suppressors, a
copper coil Ls produced about 2/3 more Rp at 100MHz than a Ni-Cr-Fe coil
Ls. Since VHF voltage amplification is a pretty much Mu x VHF-Rp, those
who want more VHF voltage amplification need a copper, or silver, Ls. If
that does not produce enough, using no VHF suppressor at all in the anode
circuit should increase Rp and VHF voltage amplification even more.
>>Too bad you don't live a bit closer. I
>>have a decapitated 8877 on the shelf that--when viewed under a
>>microscope--provides a pretty good example of early-stage gold
>>sputtering. There are five areas of gold pooling and a number of gold
>>blisters on the grid that have burst their tops and strewn gold
>>melt-balls about the cathode--as Eimacs' Mr. Foote described.
>>And.........if you get smiled-out on that one, I have another gold
>>sputtered 8877 in the garage that WE can decapitate and examine for
>>another round of laughs.
>
>I'm absoltely certain that the sputtering exists, having seen your
>photographs, both in QST and on your web pages.
There is no photo of a gold sputtered grid on my Web site.
>What I'm still not sure
>about is WHY that sputtering took place, and especially what frequency
>- VHF, HF or even DC - was responsible. All we truly know is that the
>grid has been very hot,
...snip...
Not the entire grid. The evidence typically shows that the surface of
the gold plating at the grounded end was where gold pooling and
sputtering took place.
>...and probably that there was a loud bang.
I was not present when either 8877 died.
>Similarly with the bandswitches: all we truly know is that they arced.
>They don't tell us WHY they arced, or what frequency was responsible.
RE: The QST photo of the toasted TL-922 bandswitch in "Parasitics
Revisited": This bandswitch had a measured breakdown potential of about
6000V. The anode supply was 3000V. The VHF parasitic suppressor
resistors were damaged. The bandswitch did not arc during tuneup, and
the operator was not inexperienced. IMO, either a higher than normal
frequency caused the bandswitch to arc, or the pair of 3-500Zs were
somehow suddenly able to produce over 9000w at 3815KHz.
Rich---
R. L. Measures, 805-386-3734, AG6K
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|